A MIMO scheme using a plurality of antennas in a transmitter and a receiver is at the forefront of techniques noted for having the potential to improve the performance of wireless/mobile communication systems by increasing frequency efficiency and transmission/reception diversity.
As one conventional technique of a MIMO system, there is a singular value decomposition (SVD) method. The SVD method allows a transmitter to exhibit maximum performance by singular value decomposition of complete channel environmental information fed back from a receiver. However, generally, in systems such as a frequency division duplexing (FDD) system, a transmitter should be provided with information on a channel matrix by a receiver. Since this information is quite copious, such a system cannot easily be applied to an environment in which a channel varies over time. To solve this problem, techniques of feeding back only limited channel information to a transmitter to improve performance have been researched and developed. Such techniques include a per antenna rate control (PARC) method, a per stream rate control (PSRC) method, a per unitary basis stream user rate control (PU2RC) method, and so forth.
Among these techniques, the PARC method allows a receiver to use a successive interference cancellation reception scheme and to feed back only information about signal-to-interference-plus-noise ratios (SINRs) of antennas, and thereby can obtain a theoretical open loop channel capacity of a MIMO channel.
While the PARC method allows each data stream to be separately transmitted through each antenna, the PSRC method allows each data stream to be precoded using a unitary matrix fed back from a receiving terminal and each of the precoded data streams to be transmitted. The PSRC method allows a receiver to feed back information on which of a plurality of unitary matrices capable of being used for precoding is to be used in a transmitting terminal, so that the transmitting terminal can perform precoding using each row vector of the unitary matrix and transmit each precoded data stream. The receiver also feeds back SINRs of each data stream to be precoded, and the transmitting terminal determines data transfer rates of each stream with each SINR and sends the data.
Unlike the PARC and PSRC methods which improve link performance among transmitting and receiving terminals, the PU2RC method improves performance by utilizing multi-user diversity when there are multiple terminals using multiple antennas. When a base station transmits a plurality of data streams, the PU2RC method allows each data stream to be precoded using each column vector of a unitary matrix and the precoded data streams to be transmitted, which is the same as the PSRC method. One thing different from the PSRC method is that the PU2RC method transmits each data stream to other users. According to the PU2RC method, a SINR of a data stream is fed back from each user and the data stream is allocated to a user having the best SINR, thereby improving system performance.
Both of the conventional PARC and PSRC methods of a MIMO system should uniformly maintain the transmission power of each antenna or each data stream, and thus provide less channel capacity than a method capable of adjusting transmission power. All of the methods require that all antennas or data streams have the same transmission power. When it is assumed that all antennas or data streams are allotted the same share of transmission power from a base station, a receiving terminal feeds back the SINR of a case where a successive interference cancellation scheme is used. The same power is supplied even to the antenna or data stream having the worst SINR. If the power supplied to the specific antenna or data stream is reduced or removed and additionally supplied to another antenna or data stream having a better SINR, and a transmission rate of the data stream is increased, a sum of data transmission rates can be increased. However, according to the methods, a transmitter cannot discretionarily vary transmission power because SINR information fed back to a receiver is calculated on the assumption that all antennas or data streams have the same transmission power. If a transmitting terminal varies the transmission power, the reception SINRs of each data stream cannot be known.
Compared to the PARC and PSRC methods, the PU2RC method has an advantage in that performance can be improved using multi-user diversity in a multi-user environment, but has a drawback in that interference between data streams cannot be cancelled using the successive interference cancellation scheme. On the other hand, compared to the PU2RC method, the PARC and PSRC methods can cancel interference between data streams using the successive interference cancellation scheme, but cannot improve performance using multi-user diversity in a multi-user environment. The performance of the two classified methods substantially depends on the number of users in the system and the channel environment. In general, the PU2RC method exhibits good performance in a channel environment which has a lot of users and is close to a line of sight (LOS) environment. In contrast, the PARC and PSRC methods exhibit good performance in a channel environment which has relatively few users and is close to a rich-scattering environment.